Extrude Faces Offset Snap
Set the pivot to "active element" and the transform orientation to "normal" 
Select all four faces in Edit Mode. 
Once they are selected, hit Alt+E and select "region (vertex normals)" 
While the tool is active, hold Shift (to move in smaller increments) and Alt (to turn on even thickness), now it's possible to offset the faces in a predictable way 


It moves two units each step instead of just one, but it's a workable solution. Also adjust the extrude settings to offset by just one unit: